1. Field of the Invention
The present invention relates to steering control devices.
2. Description of the Related Art
Conventionally, there are electric power steering systems (EPS) that apply motor torque serving as an assist force to a steering mechanism of a vehicle. For example, a control device of an EPS described in Japanese Patent Application Publication No. 2015-163498 (JP 2015-163498 A) calculates an assist controlled variable based on a plurality of kinds of state quantities indicating the steering state such as steering torque and a steering angle, and controls a motor based on the assist controlled variable. The control device individually sets limiting values (upper and lower limits) for each state quantity. The limiting values are the values that limit the range of variation in assist controlled variable according to each state quantity. The control device calculates the sums of these individually set limiting values to generate final limiting values for the assist controlled variable. The control device of the EPS thus has a limiting function for abnormal assist controlled variables. Accordingly, when a calculated assist controlled variable has an abnormal value, the control device limits the abnormal assist controlled variable to an appropriate value according to each state quantity by the final limiting values.
However, the EPS of JP 2015-163498 A has the following problem. In order to ensure reliability of the limiting function for the assist controlled variable, the plurality of kinds of state qualities that are used to calculate the assist controlled variable are required to have a certain level of reliability. However, depending on the specifications specified by vehicle manufacturers etc., the plurality of kinds of state quantities may include a state quantity that does not meet the required level of reliability. In this case, the control device of the EPS has to calculate the limiting values by using the state quantity that does not meet the required level of reliability. Since reliability of the limiting values calculated by using such a state quantity is not ensured, the control device may not be able to limit an assist controlled variable having an abnormal value to an appropriate intended value.